Various optical encoders are known that use a readhead having a relatively simple optical arrangement that includes a lens to provide an image of a scale pattern to a photodetector arrangement in the readhead. This type of system, which uses incoherent light to image a scale pattern onto an optical detector to thereby measure the relative or absolute displacement of the scale pattern, is called an imaging-type encoder. One such system is disclosed in U.S. Pat. No. 5,539,519, to Takagi et al., which is incorporated herein by reference. The system described in the '519 patent includes an encoder plate having a periodic slit pattern. A light source illuminates the slit pattern to form a primary fringe image. A lens projects the primary fringe image by a given magnification to form a secondary enlarged fringe image that shifts along a second plane. A fixed light receiving unit receives the shifting image through a fixed periodic mask pattern (an index grating) at the second plane. Some imaging-type optical encoder readheads are known that use a relatively simple optical arrangement that further includes a telecentric aperture. A telecentric aperture provides relatively constant magnification over a desired range of object distances.
Users generally prefer that the readheads and scales of optical encoders, including imaging-type optical encoders and displacement sensors, are as compact as possible. For example, a smaller optical encoder is more convenient to install in a variety of applications. However, reducing the size of readheads and scales generally makes their accuracy relatively less robust with respect to contamination, alignment, and other variations that may be expected in industrial environments. Thus, as the size of readheads and scales are reduced, in order to maintain accuracy as robustly as possible, each aspect of the encoder design becomes more critical, including the imaging system, the optical detector system, and the illumination system. In particular, the potential problems and improvements related to the illumination systems for small imaging-type optical encoders have been under-appreciated.